Urinary oxalate excretion in astronauts during long‐duration spaceflight: an exploratory analysis

Abstract

Objectives To explore the relationship between diet, mission phase, urinary calcium, and urinary oxalate during long‐duration spaceflight. Subjects and Methods Dietary intake of calcium and oxalate and 24‐h urine parameters from 12 astronauts during long‐duration spaceflight while consuming controlled diets were analysed. Mixed‐effects models assessed differences in urinary calcium and oxalate levels, adjusting for daily intake of calcium and oxalate, and for in‐flight status relative to pre‐flight status. Results From pre‐flight to in‐flight, median urinary calcium increased from 178.9 to 270.3 mg/day ( P < 0.01) and urinary oxalate from 37.9 to 42.3 mg/day ( P < 0.05). Post‐flight, urinary calcium decreased to 185.8 mg/day ( P < 0.01) and urinary oxalate to 31.5 mg/day ( P < 0.01), relative to in‐flight values. Dietary oxalate remained similar pre‐flight and in‐flight (382.0 vs 339.4 mg/day; P ≥ 0.05) but decreased post‐flight to 241.6 mg/day ( P < 0.01). Adjusted in‐flight urinary oxalate was 10.71 mg/day higher than pre‐flight and increased by 0.050 mg/day per 1‐mg/day increase in dietary oxalate (95% confidence interval 0.036–0.064; P < 0.001); dietary calcium was not associated with urinary oxalate ( P ≥ 0.05). Conclusions Hyperoxaluria was observed during long‐duration spaceflights and persisted throughout the in‐flight phase under controlled dietary conditions. Although high dietary oxalate was the strongest predictor, evidence of inter‐individual susceptibility was observed, with some astronauts exhibiting elevated oxalate pre‐flight levels, which increased further during flight. Assessment of oxalate content in current astronaut food systems, mitigation of dietary oxalate load through timed calcium intake, and further investigation of microgravity‐related effects on oxalate absorption and metabolism are required.